1. Field of the Invention
The present invention relates to an image forming apparatus of the electrophotographic type for copiers, printers and the like, and specifically relates to a cleanerless image forming apparatus and method.
2. Description of the Related Art
Conventional image forming apparatuses of the electrophotographic type are known to comprise a rotatable image-bearing member, charging device for charging the surface of said rotating image-bearing member to a predetermined polarity, exposure device for forming an electrostatic latent image on the surface of said image-bearing member via optical exposure in accordance with image information, developing/recovery device for simultaneously developing said electrostatic latent image and recovering untransferred developer from the surface of said image-bearing member, and a transfer device for transferring the developed image onto a transfer member.
In the aforesaid type of image forming apparatus, residual developer remaining on the image-bearing member which is not transferred to the transfer member in the previous image formation is not recovered by a special cleaner, but recovered by a developing/recovery device based on an electric field formed between said image-bearing member and said developing/recovery device to which a developing bias is applied.
Specifically, an electrostatic latent image is formed on the surface of the image-bearing member which has been uniformly charged by the charging device via optical exposure in accordance with image information by an exposure device. When this electrostatic latent image arrives at a position opposite the developing/recovery device in conjunction with the rotation of the image-bearing member, an electric field is formed between said latent image and the developing/recovery device to which a developing bias is applied such that developer accommodated in the developing/recovery device is pulled in the direction of the latent image. Thus, the developer is electrostatically adhered to the latent image so as to develop said latent image. In contrast, an electric field is formed between the developing/recovery device to which a developing bias is applied and the regions of the image-bearing member surface outside the latent image area such that untransferred developer remaining on the image-bearing member is electrostatically attracted to the developing/recovery device. The untransferred developer in the regions of the image-bearing member surface outside the latent image area is thus electrostatically attracted and recovered based on said electric field. Developing of the electrostatic latent image and recovery of the untransferred developer is accomplished simultaneously by the aforesaid developing/recovery device.
Various proposals have been made relating to cleaning devices and cleaning methods for recovering untransferred developer at times of non-image formation when electrostatic latent images are not being developed so as to improve the efficiency of recovering untransferred developer in image forming apparatuses of the aforesaid cleanerless type, and an example of such proposals is disclosed in Japanese Laid-Open Patent Application No. HEI 6-43789.
In conventional image forming apparatuses including devices such as that disclosed in the aforesaid application, the direction of the electric field between the developing/recovery device and the uniformly charged image-bearing member is set in one direction of in either a direction toward the image-bearing member from the developing/recovery device or the opposite direction to apply a direct current (DC) developing bias to the developing/recovery device during cleaning. Included among the untransferred developer, however, is developer charged to a standard polarity and developer charged to the opposite polarity. Therefore, either the residual developer of standard polarity or the opposite polarity is unrecovered during cleaning, such that unrecovered developer remains on the surface of the image-bearing member. Disadvantages arise when image formation is repeated under the aforesaid condition inasmuch as the unrecovered developer soils, for example, the charging devices of the contact type by gradual accumulation of the developer on the charging brush, thereby causing inadequate charging of the image-bearing member and background fog on the transfer member.